PALEOECOLOGY OF FOSSIL MOLLUSCAN BIVALVES, BED III, OLDUVAI GORGE, TANZANIA

The vast expanse of rock exposure at Olduvai Gorge in northern Tanzania yielded faunal records that generated syntheses on the evolution of our stone tool-using ancestors. These records emerged primarily from vertebrate, hominin and archaeological remains in Beds I, II, III and IV. Here we present macroinvertebrate fossils that contribute to paleoenvironmental interpretations of Bed III, 1.2-0.83 myr strata. Fossil mollusc bivalves, collected from Bed III at MNK site (Locality 88) in the Side Gorge, consist of complete shells, articulated and non-articulated, and angular shell fragments. A systematic investigation indicates the large freshwater fossil bivalves belong to Chambardia wahlbergi wahlbergi (Iridinidae). Discovered in fluvial channel deposits, the bivalves were distributed in or near the base of the deeper scour channel deposits. No other fossil macroinvertebrate shells or remnants are represented in these deposits. Today, the genus Chambardia consists of edentulous freshwater mussels that occur throughout Africa from the Nile Delta to South Africa. Together with the geological data, the occurrence of this fossil species is indicative of the presence of a permanent river that consisted of a river channel and a periodically inundated floodplain. The bivalves probably lived in their typical habitat, the periodically dessicated part of the river, and were swept by currents into the deeper channel and buried. It is assumed that the fluvial level changes were seasonal, indicative for a marked dry and wet seasonality in a semi-arid environment. This riverine paleoenvironmental setting stands in contrast to the Bed I and Bed II time, when an alkaline/saline lake was present and freshwater was restricted to standing groundwater-fed pools with snail species. During the ensuing Olduvai Bed III-IV times, conditions became more favorable with the instauration of a freshwater riverine system, providing perennial potable water. H. erectus skeletal remains recovered by M. D. Leakey (1971) are close to the fossil bivalve locality, and our lithostratigraphic investigation extends the riverine system interpretation to this paleoanthropological site in the Side Gorge.

Leakey, M.D., 1971, Olduvai Gorge, Excavations in Beds I & II, 1960–1963, Vol. 3. London, Cambridge, Cambridge University Press, 306 p.